FIG. 1 of the accompanying drawings illustrates a conventional RC attenuator network, such as may be used for coupling an input signal to a measuring instrument, e.g., an oscilloscope. The network comprises two resistors 2 and 4 connected in series between an input terminal 6 and ground, and two capacitors 8 and 10 connected in parallel with the resistors 2 and 4 and connected at their junction point to an output terminal 12. If the desired attenuation factor of the attenuator network is n, i.e., the amplitude of the output signal is 1/n times the amplitude of the input signal, then R.sub.2 is equal to (n-1)R.sub.4 and C.sub.8 is equal to (n-1)C.sub.10. The resistors 2 and 4 attenuate the d.c. component of the input signal, whereas the capacitors 8 and 10 attenuate the a.c. component.
It is well known to fabricate an RC attenuator network, such as that shown in FIG. 1, using thick or thin film technology. In such a case, each resistor comprises a film of resistive material deposited on a dielectric substrate, such as a ceramic material, within a predetermined boundary and extending between two spaced terminal portions of the film, at which the resistive material contacts film conductors which are also deposited on the substrate. In the case of thick film technology, the resistors and conductors are deposited on the substrate by a screen printing process using appropriate pastes. The screen printing process is also used to form the capacitors, connected to the resistors by conductors, on the substrate. The capacitance value of the capacitor 18 is trimmed or adjusted by active laser trimming. The d.c. resistance value of the resistor 2 is trimmed by passive laser trimming, which involves using a laser light beam to form a cut or kerf in the film, removing the resistive material along a predetermined line until the resistance value of the resistor attains the desired value.
FIG. 2A is a plan view of the resistor 2. The resistor has two terminal portions 2a and 2b at which it is connected to conductors 14 and 16 respectively. The resistance value of the resistor that is initially deposited on the substrate 3 is lower than the expected desired resistance value. Provided that the network has been properly formed, any departures of the d.c. properties of the network from the desired d.c. properties are attributable to the resistance of the resistor 2 being too low, and in order to bring the d.c. properties of the circuit to the desired level it is necessary only to increase the resistance value until it attains the proper level. This adjustment of the resistance value is accomplished by passive laser trimming. In accordance with this technique, a laser light beam is used to remove, by evaporation, material of the resistor along an L-shaped cut line 18 so as to increase the value of the resistance between the conductors 14 and 16. The limb 18a of the L lies wholly within the area of resistive material, while the other limb 18b extends to the boundary of the resistive material. Thus, the film is divided into two regions 20a and 20b. The region 20a includes the portions 2a and 2b and is utilized in conducting the current between the conductors 14 and 16, whereas the region 20b is not available for conduction of current between the conductors 14 and 16.
The equivalent circuit of the trimmed resistor is shown in FIG. 2B. It will be seen from FIG. 2B that the resistor 2 is composed of three resistances 22, 24 and 26 connected in series between the conductors 14 and 16, representing the area 20a, a parasitic resistance 28 connected between the resistances 24 and 26 and representing the area 20b, and a stray capacitance 30 across the laser cut and connecting the resistance 28 to the resistances 22 and 24. (The resistances 24 and 28 and the capacitance 30 are shown in distributed form.) The values of the capacitance 30 and resistance 28 (and also of the resistances 24 and 26) are dependent on the length of the laser cut 18a necessary to establish the desired d.c. resistance value.
The RC time constant of the resistance 28 and capacitance 30 causes the resistor 2 to exhibit a form of the phenomenon known as geometric hook. Hook results in a distortion of the waveform of a signal passing through the resistor. Thus, if the signal applied to the input terminal of the attenuator network has the step-form of the waveform shown in FIG. 3, geometric hook may cause the signal developed at the output terminal to have the form of the waveform b in which the portion of the step just after the rising edge is distorted from the horizontal form of the input signal. The distortion may be up to about 3% of the signal amplitude. Geometric hook in the resistor 2 cannot readily be compensated for by adjustment of the other components of the attenuator network.